Early karstification in a dual-fracture aquifer: the role of exchange flow between prominent fractures and a dense net of fissures

The early evolution of flow and hydraulic conductivity in a dual-fracture karst aquifer is investigated by numerical modelling. The initial aquifer consists of a dense net of narrow fissures with aperture widths a0 and a coarser network of prominent fractures with aperture width A0 > a0 embedded into it. Dissolutional widening by carbon dioxide containing waters aggressive to calcite increases hydraulic conductivity in the net and in the prominent fractures. To elucidate the effects of hydraulic and chemical coupling between these two flow systems the evolution of the karst aquifer has been modelled under constant head boundary conditions for various coupling strengths given by the ratio a0/A0 = M. For an isolated net of prominent fractures, a0 = 0, and M = 0 a conduit evolves along one of the percolating pathways and exhibits the well known breakthrough behaviour of single 1D conduits. With increasing coupling, M < 0.3, the breakthrough times rise from 30 ky (M = 0) to 60 ky (M = 0.3), but the pattern of the fracture widening remains almost unchanged. With further increasing M the breakthrough times drop significantly and remain constant at values of about 4 ky for M > 0.5. In these cases, owing to increasing a0, short cuts through the narrow fissures become more favourable than breakthrough along the meandering pathway of the prominent fractures. It is shown that exchange flow between the prominent fractures and the dense net of narrow fissures plays an important role in the evolution of the aquifer. Flow from prominent fractures into the net can either leave the aquifer through this net, or it can return back into a pathway of prominent fractures downstream. In the first case enhanced inflow of aggressive solution into the prominent fractures at the input increases the dissolution rates and reduces breakthrough time consequently. In the other case the solution which reenters into a prominent fracture has gained a calcium concentration closer to equilibrium than that in the prominent fracture. This reduces dissolution rates in its downstream part an retards karstification. Our model scenarios presented show a complex interplay between these two mechanisms.